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Building Tips and technical articles. => Paint and finishing => Topic started by: Motorman on November 04, 2020, 08:28:38 PM
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I made a pair of flaps using 2 pieces of 1/8 with 1/32 epoxied in the center for the hinge pockets. Came out nice and flat. Put on 2 coats of Sig nitrate then CF tissue again with nitrate. Everything looked good then I put on 2 coats of Sig buterate. I came back to it about 2 weeks later and they were warped about 3/8" not twisted just bent. Did the dope shrink more on one side than the other?
I wet down the concave side with water and weighed it down flat but it didn't do anything. Should I just put more dope on the concave side and hope it warps back straight or sand it all off and try to flatten them out with heat/water? I've got some work in these hate to just scrap them.
Also bent my Robins view stab the same way, really don't want to scrap that.
Motorman 8)
I had that happen on a set of flaps that I inherited with about 2 coats of buterate that was over 10 years old when I put on the first new coat. I was able to straighten them with a brushed on coat then hitting them with my monokote gun on low. I don't use buterate any since going electric. Personally, I have never had much luck keeping solid flaps straight for very long.
Just out of curiosity, were the top and bottom sheets from the same balsa batch? It may not have been the dope shrinking but the balsa expanding at a different rate then the dope dries and locks in the warp.
Ken
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I made a pair of flaps using 2 pieces of 1/8 with 1/32 epoxied in the center for the hinge pockets. Came out nice and flat. Put on 2 coats of Sig nitrate then CF tissue again with nitrate. Everything looked good then I put on 2 coats of Sig buterate. I came back to it about 2 weeks later and they were warped about 3/8" not twisted just bent. Did the dope shrink more on one side than the other?
I wet down the concave side with water and weighed it down flat but it didn't do anything. Should I just put more dope on the concave side and hope it warps back straight or sand it all off and try to flatten them out with heat/water? I've got some work in these hate to just scrap them.
Use Lite-Coat with about about twice the recommended quantity of plasticizer. That will allow the excess to get down into the Supercoat and loosen it up a bit. It wouldn't hurt to block it up with a little bow the other direction while it dries.
Brett
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Sometimes it's easier to just build new parts. Also, and for those that don't know, always keep the grain parallel to the leading edge.
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Sig Lite-Coat is your friend. The only time I use shrink dope is for first two coats on open bays only. From there on no-shrink. Putting shrink dope on any planking or solid surface is a good way to end up frustrated.
Peace,
Chuck
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If I ever get these things straight I'm going right for the fiberglass and epoxy.
Did you use the light-cote and plasticizer?
Brett
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I assume the dope that caused the warp was Sig Supercoat Clear? If so you must already know it shrinks a tremendous amount. I use it only for a single thinned coat on open bay areas of the wing , stab , etc. I've had the same warps as you describe I had sanded & reapplied lite coat on the outer camber side of the flap until satisfied...
John L.
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I've had great success with deft lacquer wood sealer I even applied silk with it.
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I assume the dope that caused the warp was Sig Supercoat Clear? If so you must already know it shrinks a tremendous amount. I use it only for a single thinned coat on open bay areas of the wing , stab , etc. I've had the same warps as you describe I had sanded & reapplied lite coat on the outer camber side of the flap until satisfied...
John L.
Exactly, I use nitrate, and while it looks horrible 10 minutes after you do it, all crooked, by the next day it almost always straightens out enough.
Brett
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I am
a, never using that laminating technique again
b, never using dope on flaps again
Motorman 8)
Before you say that, examine the stiffness of your flaps before dope and after. Then re-evaluate your position.
Floppy flaps can cause the most exasperating trimming issues you'll ever encounter.
You simply need to use the correct dope properly thinned.
For all-balsa flaps give the flap 2 to 3 coats of Lite-Coat thinned 50/50, sanding between coats. Apply covering with the same 50/50 Lite-Coat. Two to three more coats sanding between coats and then start with your silver. Sand the silver and re-apply until the silver comes out blemish free. Then color.
It's common for contest balsa to bow a bit on the first side covered but it will go back straight when you do the other side. Cover both sides in the same session.
Chuck
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I am
a, never using that laminating technique again
b, never using dope on flaps again
Motorman 8)
Build them up. 1/4" C/F tube LE & 1/8" CF TE Cap and 1/32 sheet. Lighter, stiffer and they never warp.
Personally, I will never use solid flaps again on a PA. Profile, maybe but since it will be electric, dope will be NT Nitrate.
Ken